Siebe’s questions:
“Are those underlying tendencies when they start to become active a kind of mano sankhara and do they evolve to vaci sankhara?”
– Those underlying tendencies are in one’s gati, asava/anusaya. Based on a sense input trigger, they could be “re-awakened” and arise as mano sankhara automatically. Then one may start consciously thinking about it (vaci sankhara), and even may do bodily actions (kaya sankhara). If one is mindful, one could stop those vaci sankhara.
“If so, are they as mano sankhara not able to cause rebirth?”
– As explained above, those gati and asava/anusaya are strengthened not by mano sankhara, but by subsequent vaci/kaya sankhara.
“Does an underlying tendency have to become active (stimulated by a sense-input) to cause rebirth, or is just the presence of an underlying tendency (anusaya) sufficient condition to cause rebirth?”
– Yes. An underlying tendency (gati, asava/anusaya) have to become active (stimulated by a sense-input) to cause rebirth. However, a particular gati, asava/anusaya is triggered by a kamma vipaka at the moment of death (or more accurately at the cuti-patisandhi moment).
More details at: “Dhamma, Sankhara, Sankata, Rupa, Vinnana, Gati, Asava, Anusaya“